The field to which our invention relates is that known as Electrical Discharge Machining, hereinafter sometimes referred to as "EDM", in which material is removed from an electrically conductive workpiece by the erosive action of electrical gap discharges between one or more electrodes and the workpiece. A servo feed system is employed to provide relative movement and thus to maintain an optimum gap spacing between electrode and workpiece as material is progressively removed from the workpiece. A dielectric coolant fluid is circulated and recirculated, generally under pressure, through the gap throughout the machining operation. For most reliable and dependable results, an independent pulse generator is utilized to provide machining power pulses of precisely controlled frequency and on-off times. One type of pulse generator suitable for use is shown and described in Oliver A. Bell, Jr., U.S. Pat. No. 3,809,847, issued on May 7, 1974 for "Method and Apparatus for Electrical Discharge Machining".
Included in the power output module are a DC power source, a plurality of electronic output switches and additional stages for taking the triggering pulses from the pulse generator, amplifying and shaping them and finally using them to drive the output electronic switches. The electronic switches shown in the exemplary embodiment are transistors. By "electronic switch" I mean any electronic control device having a plurality of electrodes comprising at least two principal or power conducting electrodes acting to control current flow in the power circuit, the conductivity of the circuit being controlled by a control electrode in the switch whereby the conductivity of the power circuit is controlled statically or electrically without the movement of any mechanical elements within the switch. Included within this definition, by way of illustration but not limitation, are vacuum tubes, transistors, semi-conductor control rectifiers, and similar devices.